The present invention relates to a method for nitriding-processing an iron group series alloy substrate (processing subject) containing an alloy element which easily forms a nitride by plasma nitriding.
Herein, Fe-base high alloy steel (containing a large amount of alloy elements) such as stainless steel, heat resistant steel (high nickel and high chromium steels) and the like is mainly explained but is not limiting. As used herein, xe2x80x9cnitriding-treatingxe2x80x9d means only treating for forming a nitrided layer, and xe2x80x9cnitriding-processingxe2x80x9d means a process of a series of steps including nitriding-treating and pretreatment such as removal of a passivated membrane and the like.
A passivated membrane means a corrosion membrane and a passivated membrane of an oxidized membrane present on the surface of a substrate.
In addition, an iron group series alloy conceptionally contains not only an iron series alloy, a representative of which is steel containing mainly iron, stainless steel, but also superalloy such as Ni-base alloy and Cr-base alloy which has a base of an iron group other than iron (old 8 group 4 series ).
The shape of a substarate conceptionally contains not only original materials such as plate material, bar material and pipe material but also engine valve, stainless products, bolt, nut, cutting tool, mold and other machinery parts which are the form of product.
In addition, a temperature (atmosphere) denotes a gas temperature unless otherwise indicated.
Nitriding-processing as a means for improving mainly the resistance to abrasion and resistance to fatigue on the surface of high alloy steel such as stainless steel and the like is well known.
As nitriding-processing, a gas nitriding method of heating a steel in an ammonia gas, a salt bath nitriding method using cyanate (tufftriding method), and a plasma nitriding method (ion nitriding method) of holding a steel in nitrogen plasma utilizing glow discharge are frequently used (see ┌Iwanami Physicochemistry Dictionary, 5th ed.┘, published by Iwanami Shoten, 1998, p.841).
In nitriding-processing, a processing subject high alloy steel is provided with a passivated membrane (including a metal oxidized membrane and a corrosion membrane:hereinafter, simply referred to as ┌passivated membrane┘) resulting from self passivation due to production of an oxidized membrane of chromium or the like, and the passivated membrane inhibits nitriding-processing and, therefore, the passivated membrane needs to be removed before nitriding treatment.
As a means for removing the passivated membrane, a method using a chlorine series or fluorine series gas is adopted in a gas nitriding method. However, since those gases are corrosive, a gas nitriding apparatus itself is corroded and, therefore, there was a problem regarding stable treatment of a large amount of products. In addition, a salt bath nitriding (tufftriding method) using cyanate becomes problematic from a viewpoint of the earth environment.
Removal treatment and nitriding treatment for this passivated membrane have to be continuously done. When there is some time between passivated membrane removal and nitriding treatment, that is, when placed in an air, a passivated membrane is regenerated on a processing subject.
When the iron group series alloy substrate is nitriding-treated by plasma nitriding, a passivated membrane can be partially removed but it is stably removed with difficulty. As a means for improving it, argon sputtering is considered to be effective because the use of an atom having a large atomic weight generally exerts the better sputtering effect. A method for placing a cleaned processing subject in a plasma nitriding furnace and, thereafter, sputtering oxygen atoms by argon sputtering to remove a passivated membrane and performing nitriding has been utilized by some investigators.
However, it was found that this removing method can not remove a passivated membrane fully and stably in plasma nitriding processing. When removal of a passivated membrane is insufficient, it is difficult to obtain a nitrided layer of good quality by nitriding treatment. That is, it is difficult to obtain the required hardness of the surface and, at the same time, scatter of the surface hardness in a nitrided layer tends to occur.
In austenitic and precipitation hardening stainless steels and high alloy steels and superalloys containing a large amount of Cr, a firm passivated membrane is easily produced. For this reason, concerning these high alloy steels, it has been considered that a nitrided layer of stable quality is hardly obtained by nitriding treatment.
In addition, since the aforementioned argon sputtering can not be stably performed until a predetermined temperature (normally, a temperature near plasma nitriding treatment, for example 350xc2x0 C.) is attained, it needs a time to raise a temperature to a temperature optimal for argon sputtering and, thus, nitriding-processing as a whole tends to need a longer time (see FIG. 1).
Further, a slight amount of water steam (H2O) is usually contained in a nitrogen gas and an argon gas. The H2O is ionized by glow discharge in a plasma nitriding furnace, (hereinafter, simply referred to as ┌nitriding furnace┘ in some cases) and active oxygen is generated. The active oxygen contributes to regeneration of a passivated membrane and may inhibit removal of a passivated membrane.
In view of above circumstances, an object of the present invention is to provide a method for nitriding-processing an iron group series alloy substrate which can sufficiently remove a passivated membrane upon plasma nitriding of an iron group series alloy substrate and, as a whole, can assurely perform uniform nitriding-processing and, further, can perform nitriding-processing in a short period of time.
In view of the above circumstances, the present inventors studied intensively and found an unexpectedly high quality method for nitriding-processing an iron group series alloy substrate by plasma nitriding treatment, which has the essential features below and which can assurely remove a passivated membrane by hydrogen sputtering at a lower temperature range.
In a processing method for nitriding an iron group series alloy substrate (processing subject) by plasma nitriding treatment after pretreatment including passivated membrane removing treatment, said method comprises performing passivated membrane removing treatment by hydrogen sputtering under reduced pressure atmosphere.
It is desirable from a viewpoint of passivated membrane removability that the hydrogen sputtering is performed usually in the atmosphere at a temperature below 350xc2x0 C., desirably in the atmosphere at a temperature below about 150xc2x0 C.
It is more desirable from a viewpoint of quality stability and productivity that hydrogen sputtering is performed in the atmosphere at a temperature below 150xc2x0 C. (usually normal temperature) in a process of raising a temperature of a processing subject in a plasma nitriding furnace.
It is desirable that a nitrided layer is formed at least initially by plasma nitriding treatment at a treatment temperature of 350 to 450xc2x0 C. because a nitrided layer of stable quality is obtained when applied to austenitic stainless steel and the like.
In addition, it is desirable that the present nitriding-processing method is applied to an iron group series alloy substrate containing 3 wt % or more Cr as an alloy element because the effects of the present invention become remarkable.